fix(labels): replace incorrect ams_30x15 preset with correct AMS holder sizes (#1426)

Reporter — the same person who originally requested the labels
  feature in #809 — discovered that the ams_30x15 preset's 30x15 mm
  dimension didn't actually fit any variant of the MakerWorld AMS
  Filament Label Holder (model 752566) it advertised. Two new
  presets replace it:

  - ams_holder_74x33 (74 x 33 mm) matches the printable label STL
    bundled in the MakerWorld project
  - ams_holder_75x55 (75 x 55 mm) fits the cardstock-insert variant
    the reporter validated on bench

  Both cross the 20 mm height threshold so they land in the roomy
  layout branch — swatch on the left, QR on the right, multi-line
  text (brand, material, hex code, spool ID) in the middle. The
  old 30x15 mm preset couldn't fit a QR code; the new ones do.

  No DB migration: the preset name was never persisted. Callers
  scripting the old ams_30x15 value get a clean 422 at the route's
  Literal validator with the new valid values listed.

  i18n: replaced inventory.labels.templates.ams.{label,hint} with
  amsHolderSmall and amsHolderLarge across all 8 locales with real
  translations; parity guard cleaned of the stale English-fallback
  cognate entries. Parity holds at 4856 leaves per locale.

  Tests: backend label renderer + integration tests cover both new
  presets; LabelTemplatePickerModal test updated for the 6-button
  grid and the new template value in the API-call assertion.

backend/app/api/routes/labels.py

@@ -37,7 +37,8 @@ logger = logging.getLogger(__name__)
 router = APIRouter(tags=["labels"])
 
 _VALID_TEMPLATES: tuple[TemplateName, ...] = (
-    "ams_30x15",
+    "ams_holder_74x33",
+    "ams_holder_75x55",
     "box_40x30",
     "box_62x29",
     "avery_5160",
@@ -51,7 +52,14 @@ MAX_LABELS_PER_REQUEST = 500
 
 class LabelRequest(BaseModel):
     spool_ids: list[int] = Field(..., min_length=1, max_length=MAX_LABELS_PER_REQUEST)
-    template: Literal["ams_30x15", "box_40x30", "box_62x29", "avery_5160", "avery_l7160"]
+    template: Literal[
+        "ams_holder_74x33",
+        "ams_holder_75x55",
+        "box_40x30",
+        "box_62x29",
+        "avery_5160",
+        "avery_l7160",
+    ]
 
 
 def _split_extra_colors(raw: str | None) -> list[str] | None: